In cells or tissues different in their functions or properties, genes with different expression level exist, and there is a possibility that they play important role in the functional analysis of cells or tissues. It is useful to obtain such genes for elucidation of their functions and expression mechanisms.
On the other hand, a method for cloning genes with different expression levels between cells or tissues with different functions or properties is known as cDNA subtraction method.
The cDNA subtraction method is performed by the following procedures.
For example, in case of analyzing specific function and properties of cancer cell tissues:
(1) At first, a single-stranded RNA or cDNA (tester) prepared from cancer cell tissues and excess amount of single-stranded RNA or cDNA prepared from normal cell tissues, or double-stranded cDNA (driver) are subjected to hybridization. At this time, the gene expressing both Tester and Driver together (housekeeping gene, etc.) forms double-stranded cDNA or RNA-DNA hybrid, however, RNA or cDNA derived from gene specifically expressing in Tester does not form hybrid, and is remained in a state of single-stranded RNA or cDNA.(2) Subsequently, the single-stranded RNA or cDNA derived from gene, which is specifically expressed in tester and does not form hybrid, is separated from the hybridized double-stranded cDNA or RNA-DNA hybrid to concentrate the genes which is tester-specifically expressed, for example, by the hydroxyapatite chromatography (Hendrick, S. M., Cohen, D. I., Nielsen, E. L. and Davis, M. M. (1984) Nature. 308, 149-153), Avidin-Biotin binding method (Duguin, J. R. and Dinauer, M. C. (1990) Nucleic Acid Research. 18, 2789-2792, JP-A-9-507021), oligo (dT)30 solid phase method (Hara, E., Kato T., Nakada, S., Sekiya, L. and Oda, K., (1991) Nucleic Acid Research. 19, 7097-7104 JP-A-2001-269172), Avidin-Biotin-Magnetic Beads method (JP-A-2001-269172 and JP-A-2002-253237), and the like.
However, since the above methods utilized physical binding or adsorption, and separated the single-stranded RNA or cDNA derived from tester-specifically expressing gene and the hybridized double-stranded cDNA or the RNA-DNA hybrid, these materials could not be properly separated, and there was a problem that admixing the hybridized double-stranded cDNA or RNA-DNA hybrid might be admixed. As a result, in the above methods, frequent contamination of the expressed genes in tester and driver (for example, housekeeping gene) might occur, consequently the tester-specific expressing genes could not be concentrated (obtained) efficiently. In addition, the above methods are complicated in operation with a large number of working steps, consequently there was a problem that time and skill are required.